<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var m = parseInt(prompt('输入月份'))
        function run(y) {
            if (y % 100 == 0) {
                if (y % 400 == 0) {
                    return true
                } else {
                    return false
                }
            } else {
                if ( y % 4 == 0) {
                    return true
                } else {
                    return false
                }
            }
        }
        if (m == 2) {
            var y = parseInt(prompt('请输入年份'))
            if (run(y)) {
                alert('29')
            } else {
                alert('28')
            }
        } else {
            if ([1,3,5,7,8,10,12].includes(m)) {
                alert('31')
            } else {
                alert('30')
            }
        }

    </script>
</body>
</html>